Smog -- also known as particulate air pollution -- is made up of tiny particles that can easily travel into the lungs.

For the study, researchers analyzed data from more than 100,000 people in Europe with no history of heart disease who were followed for an average of 11.5 years. During the follow-up period, more than 5,100 of the participants had coronary events such as heart attack or angina.

After accounting for several other risk factors such as smoking and other health problems, the researchers concluded that a 5 micrograms per cubic meter (mcg/m3) increase in particulate matter with a diameter of 2.5 micrometers or less (PM2.5) was associated with a 13 percent increased risk of coronary events. Meanwhile, a 10 mcg/m3 increase in larger PM10 particulate matter was associated with a 12 percent increased risk of coronary events.

The study was published Jan. 21 in the online edition of the BMJ.

Current annual limits for PM2.5 are 25 mcg/m3 in the European Union (EU) and 12 mcg/m3 in the United States, according to a BMJ news release. The World Health Organization's recommended limit is 10 mcg/m3.

"Our study suggests an association between long-term exposure to particulate matter and incidence of coronary events," wrote Giulia Cesaroni, a senior researcher in the epidemiology department at Lazio Regional Health Service, in Rome, Italy, and colleagues.

The authors added that their findings support the lowering of EU limits on particulate air pollution to protect public health.

Although the study found an association between long-term exposure to smog and risk of heart conditions, it did not prove a cause-and-effect relationship.